Concrete composition

ABSTRACT

Concrete articles of improved strength are prepared by impregnating the articles with aqueous solutions of alkali metal or alkaline earth metal polysulfides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates to portland cement concrete articles ofincreased strength prepared by impregnating the articles with aqueoussolutions of alkali metal or alkaline earth metal polysulfides.

Portland cement concrete has become the world's largest bulk buildingmaterial. The concrete comprises an admixture of portland cement withsand and aggregate (gravel or crushed stone). It is generally formed bymixing portland cement, water, sand and gravel or crushed stone, andallowing the mixture to harden to form a dense structure. The characterof the "paste" mixture of water and cement is generally held todetermine the important engineering properties of the concrete.

Construction concrete is generally formulated to contain from about 5 to30 weight percent cement based upon the dry weight of materials.Generally, higher cement content (in the upper range of the usualcontent) produces concrete of higher strength.

Recently, shortages of cement in many parts of the world makeparticularly attractive the prospects of supplying concrete of lowcement content which has high strength.

2. Description of the Prior Art

Polymer cements and polymer-impregnated concretes are known. The formerusually contain a latex such as polyvinyl chloride, polyvinyl acetate,acrylonitrite-butadiene-styrene, etc., or an acrylic in solid or liquidform.

U.S. Pat. No. 2,820,713 discloses mortar compositions containing minoramounts of methyl cellulose and alkaline earth polysulfides and/or buntesalts as adhesion improvers.

U.S. Pat. Nos. 2,962,467 and 3,198,644 disclose similar mortarcompositions.

"Characteristics of Portland Cement from the Gypsum-Sulfuric AcidProcess" by Fredrich Wolfe and Joachim Hille (Silikattechnik, 1967,1812), pages 55-57, discusses the effect on certain portland cements bythe addition of calcium sulfide (Ca₂ S).

SUMMARY OF THE INVENTION

It has now been found that strength of portland cement concrete objectscan be significantly improved by impregnating the porous concretestructure with an aqueous solution of an alkali metal or alkaline earthmetal polysulfide and allowing the concrete article to dry.

In the present application, the concrete structure is formed byconventional means and allowed to dry sufficiently to allowincorporation of the polysulfide solution into the interstices of thestructure. Of course, liquid-resistant concrete will not absorbsufficient polysulfide to affect the strength of the product. The porousconcretes usually contain a high ratio of coarse aggregate to fineaggregate and sand. They thus set leaving many voids and leaving themsusceptible to water absorption and make them more susceptible to theeffects of weathering, freeze-thaw cycling, etc. Thus, the presentprocess offers an inexpensive method of strengthening porous concretes.In the past, polymer-filled concretes have been known.

The polymer-impregnated concretes previously referred to are materialswherein relatively expensive organic polymers are formed in situ by thepolymerization of monomers such as styrene, etc. These materials, whilehaving excellent strength, are quite expensive, due to the monomer cost.The present invention, which employs inexpensive polysulfide solutions,allows substantial concrete strengthening and porosity reduction at lowcost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A companion application, U.S. Ser. No. 938,665, filed Aug. 31, 1978,shows that increased strength results from the incorporation in the mixof low-cement-content concrete of minor portions of alkali metal oralkaline earth metal polysulfides.

The alkali metal or alkaline earth metal polysulfide, preferably calciumpolysulfide, is employed in aqueous solution. The calcium polysulfidesare articles of commerce and are conventionally prepared by the reactionof sulfur with lime water. Other polysulfides such as barium polysulfideare produced in a similar manner. Their preparation is described on page63 of "A Text-Book of Organic Chemistry", Vol. III, Part I, edited by J.Newton Friend, Charles Griffin and Co., London, 1925. For agriculturalpurposes, they are conventionally sold as concentrated aqueoussolutions, e.g., about 30 weight percent CaS_(x).

The aqueous solutions of the polysulfide will usually contain from about5 to 30 weight percent by weight of the polysulfide, preferably fromabout 20 to 30 weight percent, in order to accomplish maximumimpregnation with the polysulfide.

The impregnation of the articles is accomplished by capillary action orby pressure or vacuum methods or combinations of these methods. In thecase of large articles, it may be only practical to spray the surface(usually continuously for extended periods), allowing the solution tointerpenetrate the structure to the maximum extent possible. Withsmaller structures such as building blocks, pipe sections, etc., vacuumpenetration is suitably employed. In these, the article is placed in acontainer with a quantity of polysulfide solution, the container issealed and vacuum is applied, resulting in removal of the entrained airin the concrete. Relaxation of the vacuum and restoration of atmosphericpressure or application of superatmospheric pressure results inimpregnation of the article with the polysulfide solution.

Any type of portland cement concrete structure or article can beemployed, i.e., in addition to conventional construction concrete,portland-porzolan cement (concrete) clay cements which employ blastfurnace slag as aggregate, expansive cement, etc., can be used.

Since the article used will usually be made of concrete which hasalready been cured before the polysulfide impregnation, wet cure isusually not required, and the article can be used immediately. It willdevelop further strength as the water evaporates from the polysulfidesolution.

The major advantage of the present process and articles made with theprocess compared with the polymer-impregnated concrete and itspreparation is that the polysulfides are inexpensive compared withorganic polymer monomers employed with those materials. Further, thepolysulfide imparts substantial strength to the concrete without heat,radiation cure or chemical catalysis necessary with the polymer.

EXAMPLES

The following examples illustrate the invention. The examples areillustrative only and are non-limiting.

EXAMPLE 1

Commercial portland concrete blocks purchased from a building supplyhouse were cut into 2-inch cubes and soaked for 5 minutes in (A) water,(B) 15% aqueous calcium polysulfide solution, and (C) 29% aqueouscalcium polysulfide solution. The cubes were removed from the soakingtanks and allowed to cure in a constant-humidity room for 7 days. Theywere then crushed in a Baldwin press to determine compressive strengthat the breaking point. The following average strengths were obtained:(A) 8966 lbs, (B) 9866 lbs, and (C) 10,033 lbs, indicating a significantincrease in strength obtained with the polysulfide solutions.

EXAMPLE 2

Following the general procedure of Example 1, but allowing the cubes tosoak in the solutions for 7 days and crushing them immediately afterremoval, strengths of (A) 8136 lbs, (B) 9461 lbs, and (C) 9800 lbs wereobtained, indicating that some strength increase takes place duringsoaking.

EXAMPLE 3

Following the general procedures of Examples 1 and 2, cubes were soakedin water, 14.5 weight percent CaS_(x) solution and 29.5 weight percentCaS_(x) solution. The data acquired were as follows:

                  TABLE                                                           ______________________________________                                                       Compressive Strength, psi                                                       (A)     (B)       (C)                                                                 14.5% CaS.sub.x                                                                         29% CaS.sub.x                              Test             Water   Solution  Solution                                   ______________________________________                                        #1, soaked 5 min., cured 7                                                    days at 50% humidity, 73° F.                                                            2242    2467      2508                                       #2, soaked 1 hour, cured 7                                                    days at 50% humidity               2717                                       #3, soaked 7 days, crushed                                                    immediately      2034    2367      2450                                       ______________________________________                                    

These data indicate that soaking time of 1 hour develops significantlymore strength than 5 minutes. This, of course, is influenced by the sizeand porosity of the concrete object. For larger objects, longer soakingtimes are necessary.

Although many specific embodiments of the invention have been describedin detail, it should be understood that the invention is to be given thebroadest possible interpretation within the terms of the followingclaims.

What is claimed is:
 1. A process for producing a portland cement concrete article of increased strength by impregnating the article with an aqueous solution of an alkali metal or alkaline earth metal polysulfide.
 2. The process of claim 1 in which the polysulfide is calcium polysulfide.
 3. The process of claim 1 wherein the impregnated article is allowed to cure by drying.
 4. The process of claim 1 in which the solution contains from 15 to 30 weight percent of the polysulfide.
 5. The process of claim 1 wherein the impregnation is carried out by vacuum impregnation. 